Switching power converters convert energy from an electrical source, such as an AC wall plug, to a form needed by an electrical load, such as a computer, appliance, or electronic circuit. Switching action provides very high efficiency, but introduces ripple and electrical noise.
In commercially available converters, filter networks built from capacitors and inductors must be used to bring ripple to a tolerable level. In some circuits, especially those rated below 5 V, effective filters are very difficult to build with affordable components.
Known switching power converters use one or two types of filter arrangements. Passive filters are built from inductors and capacitors. These are relatively large, since the components must store sufficient energy to offset ripple and noise. The inherent series resistance inside any real capacitor or inductor makes it almost impossible to meet the needs of very demanding applications.
Passive filters trade off quality for dynamic performance. It is well known in the field that switching converters rarely provide output ripple below 50 mV. This is 1% of the nominal output for a 5 V supply, and much more for emerging 3.3 V and 2 V applications. or applications demanding battery quality, so-called "linear supplies" are often used. These are much less efficient and far larger than switching supplies of similar ratings.
Active filters add an electronic circuit that modifies the output to reduce ripple. The most common active filters are the series type as illustrated in FIG. 1A. A transistor coupled in series with the output actively adjusts the voltage presented to the load.
Series active filters offer excellent performance, but sacrifice efficiency. In a typical 5 V application of a series active filter, the overall converter efficiency is reduced by nearly 30%. In a 2 V application, 50% reduction is likely. In spite of these extra losses, series active filters are widely used in the marketplace because of their high performance.
An alternative active filter arrangement uses a parallel connection. In this case, an amplifier injects a compensation current into the load. Known active filters use this method with output feedback.
Output feedback has limited ability to cancel ripple, and gives rise to stability problems. Most important, the more effective a feedback-based filter becomes, the more gain it requires to function. This inherently trades off stability and performance.
In FIG. 1A, the entire output current flows through the active element. A known parallel arrangement, shown in FIG. 1B, injects a compensation current into the output node. The series active filter is familiar and has wide commercial use, and provides performance similar to that of a linear power supply. The series voltage drop sacrifices efficiency, but the method offers good output behavior when current levels are not high.
One straightforward output active filter approach is to use output voltage feedback to drive a compensation amplifier or switching converter. The output is compared to a reference value, and proportional-integral loops are used to provide ripple correction (as in FIG. 1B).
Ripple cancellation has been accomplished for a buck converter using a linear amplifier to inject the compensation current into the output node. This approach has inherent stability problems. The load impedance is often unknown and variable.
If the ripple voltage is on the order of noise levels in the system, it is hard to sense the output ripple with sufficient accuracy. These drawbacks mean that an active filter stable under a given set of conditions is not guaranteed to be stable under changing loads or when noise levels are high.
Active filter approaches for DC-DC converters, as described in the literature, do not take advantage of knowledge of the ripple behavior. For example, the ideal current ripple in a simple buck converter is triangular, while the output ripple in a resonant converter is sinusoidal.
Thus, there continues to be a need for filters usable to improve the performance of converter circuits. Preferably, knowledge of ripple behavior could be incorporated into such filters so as to improve converter performance. In addition, it would be desirable if such filters did not substantially increase the cost, size or weight of converter circuits.